The present invention relates to gas absorption wherein light hydrocarbons are scrubbed from gaseous mixtures with a physical scrubbing agent.
It is conventional to separate C.sub.2+ hydrocarbons from gaseous mixtures, such as natural gas, by the so-called Expander process. In this process, the precooled gas, after separation of condensate, is isentropically expanded in an expansion turbine. During this step, the cooling requirements for the condensation of C.sub.2+ hydrocarbons from the gaseous mixture are satisfied at least in part by the cold-producing expansion of the gas.
However, if unfavorable marginal conditions occur, such as, for example, low concentrations of C.sub.2+ hydrocarbons or simultaneously a higher content of CO.sub.2 in the crude gas, then, alternatively, physical scrubbing procedures are also suitable for recovering the C.sub.2+ fraction. Examples of known scrubbing media for such scrubbing operations are various polyethylene glycol dialkyl ethers (PGE), N-methylpyrrolidone (NMP), dimethylformamide (DMF), propylene carbonate, or sulfolane. Disadvantageous to the use of these known scrubbing media is that, besides the hydrocarbons, sour gases (e.g. CO.sub.2, H.sub.2 S, COS) are likewise dissolved with relative ease, which gases are in most cases present in the crude gas. At the same time the solubility of C.sub.2+ hydrocarbons is very low. Under practical conditions, this means that, besides C.sub.2+ hydrocarbons, a considerable portion of the CO.sub.2 present in the gas is also scrubbed out and the amount of the circulated solvent is relatively large. In order to produce a C.sub.2+ -fraction suitable for further processing, two different ways for the recovery of this fraction from the loaded solvent are possible. The first way, normally used in oil scrubbing systems, was distillation, thereby separating the solvent as bottoms product and the absorbed gases as the overhead stream. Thereafter, this overhead stream had to be separated into a C.sub.2+ -fraction and a gas containing CO.sub.2, CH.sub.4 and lighter components like inert gases such as H.sub.2, N.sub.2 and CO. The second way is the expansion of the loaded solvent to low pressure. The flash gas is compressed and fed to a rectification column for the further recovery and purification of the C.sub.2+ product. In the rectification of the desorbed gas into methane as the overhead product and residual C.sub.2+ hydrocarbons as the bottoms product, problems are also encountered due to the tendency to form an azeotrope between CO.sub.2 and ethane.